Multiple-Parameter Radar Observations of Isolated Florida Thunderstorms during the Onset of Electrification

A. R. Jameson Applied Research Corporation, Landover, Maryland

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M. J. Murphy Institute of Atmospheric Physics, The University of Arizona, Tucson, Arizona

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E. P. Krider Institute of Atmospheric Physics, The University of Arizona, Tucson, Arizona

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Abstract

A prime objective of the Convection and Precipitation/Electrification experiment was to Study the electrification of Florida thunderstorms in greater depth using the National Aeronautics and Space Administration/United States Air Force electric field mill network; a small fleet of aircraft; the dual-frequency, dual-polarization CP-2 radar of the National Center for Atmospheric Research (NCAR), and other radars including the NCAR Doppler CP-3 and CP-4. Analyses of three small, isolated storms on 19 July 1991 suggest that the onset of electrification coincides with the appearance of significant volumes of differential reflectivity, indicative of liquid raindrops larger than 2-mm diameter, above the −7°C level accompanied by the nearly simultaneous appearance of significant depolarization, initially associated with the freezing of these drops.

Although the relationship of the onset of electrification to various aspects of the radar reflectivity factor Z are more ambiguous, a rapid increase in the altitude and magnitude of Z above the −7°C height indicates that storm electrification is under way. The polarization measurements appear to be useful for detecting conditions favorable for the onset of electrification. These conditions include the introduction of ice processes in tropical cumulonimbi that are initially dominated by warm rain processes. The freezing of supercooled drops not only produces the first graupel, but the conditions associated with this freezing likely lead, through ice multiplication and/or condensation freezing, to the rapid production of smaller ice particles thought to be important to the subsequent onset of electrification.

Abstract

A prime objective of the Convection and Precipitation/Electrification experiment was to Study the electrification of Florida thunderstorms in greater depth using the National Aeronautics and Space Administration/United States Air Force electric field mill network; a small fleet of aircraft; the dual-frequency, dual-polarization CP-2 radar of the National Center for Atmospheric Research (NCAR), and other radars including the NCAR Doppler CP-3 and CP-4. Analyses of three small, isolated storms on 19 July 1991 suggest that the onset of electrification coincides with the appearance of significant volumes of differential reflectivity, indicative of liquid raindrops larger than 2-mm diameter, above the −7°C level accompanied by the nearly simultaneous appearance of significant depolarization, initially associated with the freezing of these drops.

Although the relationship of the onset of electrification to various aspects of the radar reflectivity factor Z are more ambiguous, a rapid increase in the altitude and magnitude of Z above the −7°C height indicates that storm electrification is under way. The polarization measurements appear to be useful for detecting conditions favorable for the onset of electrification. These conditions include the introduction of ice processes in tropical cumulonimbi that are initially dominated by warm rain processes. The freezing of supercooled drops not only produces the first graupel, but the conditions associated with this freezing likely lead, through ice multiplication and/or condensation freezing, to the rapid production of smaller ice particles thought to be important to the subsequent onset of electrification.

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